The present invention relates, in general, to a method and a device for determining a rotary oscillation of a load of a lifting gear.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
When a load, for example a container, is lifted by cables, a rotation and/or wobble motion can occur, also referred to as skew. Such skew motions, which can be viewed as a certain type of rotary oscillation, are currently captured by a system with two cameras that determine the position of the load. In the following, the term “rotary oscillation” will be used in a generic sense and is used synonymously and interchangeably with rotation, wobble and/or skew motion. FIG. 3 shows a system with two cameras 8, whereby an active light source 16, for example an infrared light beam, is associated with each camera 8. Each camera 8 can capture an image field 11 with markers 9, 10. The motion during a rotary oscillation can be determined by measuring the position changes of the markers 9, 10 about a center of mass 17 of a load. The markers 9, 10 are differentiated on the load and/or the load-carrying member by applying the marker 9 to the load at an offset at a right angle relative to the marker 10. A processor evaluates the images recorded by the two cameras 8.
The use of two cameras is very expensive, so that the use of a single camera is proposed. However, current technology makes it difficult to use only one camera because the available computer power limits the time for evaluating the images recorded by the camera, so that the actual position values of the load cannot be updated and supplied fast enough to a control system for controlling the rotary oscillation.
German Pat. No. DE 4190587 C1 describes a system with only a single camera. A unit evaluates the image acquired by the camera to determine the position of a load. The image recorded by the camera has at least two markers that are used for computing the position. A system of this type suffers shortcomings because of the inability to adequately detect a rotation and/or wobble motion, also referred to as skew, of a load lifted with cables, for example a container since the image acquisition time for determining the position is very long. Measurements of the actual position values of the load are therefore too infrequent so as to be useful in a controller to counteract the skew. The limited computer power typically prevents acquisition of a greater number of actual values of the position of the load.
It would therefore be desirable and advantageous to provide an improved method and device that can cost-effectively record a rotation and/or wobble motion of a load suspended from a lifting gear.